Monomers, such as unsaturated carboxylic acids and unsaturated nitrites, are industrially important as starting and intermediate materials for producing various synthetic resins, coating materials, fibers, plasticizers, synthetic resins, and the like. Commercially, there are various processes for producing unsaturated carboxylic acids (e.g., acrylic acid and methacrylic acid) and unsaturated nitriles (e.g., acrylonitrile and methacrylonitrile), but broadly, all such processes begin in essentially the same manner, via the catalytic reaction of one or more hydrocarbons to produce an impure gaseous material comprising the desired monomer. It is then necessary to recover the desired monomer from the impure gaseous material, and then to further purify it in order to minimize the amount of other materials, by-products, and impurities present in the recovered monomer stream.
For example, one well-known and commercially successful process for acrylic acid manufacture involves a two-step vapor phase catalytic oxidation reaction wherein propylene is converted first to acrolein in a first process step, and the acrolein is then converted to acrylic acid in a second process step. The resulting gaseous material stream comprises acrylic acid, but also water, unreacted propylene and acrolein, and several by-products including, but not limited to, one or more of the following compounds: formaldehyde, acetic acid, propionic acid, benzaldehyde, furfural, and maleic acid. Another known process which is currently being explored and developed is the single step vapor phase catalytic oxidation of propane in the presence of a suitable catalyst, and which also produces a gaseous material stream comprising acrylic acid, water, carbon oxides, unreacted propane, propylene, acrolein and several by-products including, but not limited to, one or more of the following compounds: formaldehyde, acetic acid, propionic acid, benzaldehyde, furfural, and maleic acid.
Regardless of the reaction process which produces it, aqueous acrylic acid is then most typically recovered from the raw gaseous product stream in an absorption tower, wherein a cooled absorbent, such as water or an organic compound (e.g., diphenyl ether) directly contacts the gaseous material stream, simultaneously condensing and absorbing various components, including acrylic acid and water, from the gaseous material stream to produce an aqueous acrylic acid stream. This aqueous mixture typically contains appreciable amounts of by-products and impurities such as acrolein, formaldehyde, and water. This is, at least in part, because direct-contact absorption processes also capture by-products and impurities with the (meth)acrylic acid from the gaseous (meth)acrylic acid-containing material, rather than only the desired (meth)acrylic acid product. For this reason, it is common for the resulting aqueous acrylic acid stream to be subjected to one or more purification steps. The intended use for the acrylic acid will often determine the degree to which the material stream must be purified and the extent to which the other materials must be removed or separated from the acrylic acid.
Purification, or separation, of the desired acrylic acid product from other materials in the aqueous acrylic acid stream may be accomplished by one or more well-known and understood processes including distillation, extraction, and/or crystallization. One of the most common of these purification steps is the use of azeotropic distillation to remove water from the aqueous acrylic acid stream. The purification of aqueous acrylic acid streams via azeotropic distillation is well known in the art of acrylic acid production. Various azeotropic distillation processes have been developed over the years based upon different azeotropic agents, for example U.S. Pat. No. 3,798,264 teaches the use of isobutyl acetate (IBAc) as the azeotropic agent, GB Patent No. 2146636 teaches the use of methyl isobutyl ketone (MIBK) as the azeotropic agent, and U.S. Pat. No. 6,399,817 teaches the use of toluene as the azeotropic agent.
Further purification processes, such as multiple distillation steps in series, or crystallization processes, are often necessary to meet final product quality requirements. Of course, each additional purification step requires additional initial capital investment, as well as higher operational costs.
For example, the process disclosed in U.S. Pat. No. 6,482,981 involves absorption of (meth)acrylic acid by direct contact of the material stream with water, followed by azeotropic distillation, and then by crystallization of the resulting aqueous acrylic acid to remove additional impurities and thereby minimize polymerization and formation of other solids. Such a complex process is therefore economically unattractive to construct and operate.
It is important to note that some of the by-products and impurities in the aqueous acrylic acid stream may interfere with the operation and efficiency of downstream processes, such as further purification, storage, or reaction to produce other materials (e.g., esters of (meth)acrylic acids). For example, as explained in U.S. Patent Application Publication No. US 2007/0167650, high levels of formaldehyde in acrylic acid streams interact adversely with some polymerization inhibitors (e.g., phenothiazine (“PTZ”), hydroquinone (“HQ”), and monomethyl ether of hydroquinone (MeHQ”)) resulting in formation of solid precipitates in the acrylic acid product. This reaction also reduces the effectiveness of the inhibitor molecules in preventing polymerization of the acrylic acid monomer, since some of the inhibitor is consumed in forming the solid precipitates. Further, these effects worsen with increasing temperatures, such as are often used in azeotropic distillation columns. It would therefore be advantageous to remove as much of these impurities as possible at the recovery step, prior to downstream purification of the aqueous acrylic acid. Unfortunately, the disclosure of U.S. Patent Application Publication No. US 2007/0167650 does not provide guidance for removing such impurities. Instead, the application only proposes that one limit, i.e., reduce, the concentration of inhibitors in the process to avoid the formation of precipitates. Such an approach is clearly at odds with reliable process operation, as it obtains reduced precipitate formation at the cost of insufficient process inhibition.
As an alternative to conventional absorption with water or organic absorbents, U.S. Pat. No. 6,646,161 describes a process for recovering (meth)acrylic acid from a hot gas containing (meth)acrylic acid and a high proportion of non-condensable constituents, by “fractional condensation” of the hot gas. The desired aqueous acrylic acid product (having greater than 95% by weight acrylic acid) exits from the side of the fractional condenser, while light ends (uncondensed components) exit from the top and the heavier impurities and by-products exit as a condensed liquid stream from the bottom. Thus, the process of U.S. Pat. No. 6,646,161 produces an aqueous acrylic acid product, and at least two by-product streams which must be handled separately. Optionally, a wastewater stream may be withdrawn from the side of the fractional condenser, further increasing the acrylic acid content of the aqueous acrylic acid side stream, but also creating a third by-product stream which must be processed. The fractional condenser used in this process is divided into “sections” to solve various engineering problems (i.e., separation of multiple components whose boiling points differ by greater than 25° C.) and accomplishes simultaneous cooling of the hot gas and condensation of the higher boiling fraction thereof. Thus, this process may be considered a combination of rectification and absorption which is often accomplished in two separate steps in conventional processes. The benefits of obtaining a purified acrylic acid stream from such a process are outweighed, however, by the substantial complexity of having multiple streams to process and the incremental capital cost of the additional process equipment required for such an operating approach.
European Patent No. EP 1319648 B1 presents a process for producing acrylic acid wherein the effluent of a two-stage propylene oxidation process is first fed to a quench tower, wherein acrylic acid is captured via direct contact with process water. The resulting aqueous acrylic acid stream is then subjected to azeotropic distillation to remove wastewater from the aqueous acrylic acid, and the wastewater is recycled to the top of the quenching tower. A second aqueous acrylic acid stream is also withdrawn from the upper section of the quench tower and distilled; the removed water from this stream is also recycled to the top of the quench tower. Thus, this process involves withdrawing multiple product streams, both of which comprise water and acrylic acid, and both of which require further processing to produce acrylic acid of the commercially useful purity.
Industry would welcome processes which produce aqueous acrylic acid with less impurities and by-products so as to minimize problems in downstream purification steps involving accumulation of foulants, such as precipitates and polymer solids, which interfere with equipment performance and reduce the amount of inhibitor available for preventing polymerization of the (meth)acrylic acid product during later stages of processing, including purification, transport and storage. It is also desirable for such processes to be as simple as possible, i.e., to require as few process steps and as little equipment as possible, to thereby minimize capital and operating expenses.